Electrostatic prism

ABSTRACT

An electrostatic field to manipulate charged particles in a narrow band filter prism and energy discrimination prism arrangement is obtained by employing high transparency opposing screen electrodes. Adjacent each screen electrode particle attractor apparatus act to attract all particles outside the narrow band of particles. In the narrow band filter arrangement collector apparatus are provided to collect particles within the narrow band while in the discrimination arrangement collector apparatus are provided to collect both particles having energies above the narrow band and particles having energies below the narrow band.

United States Patent Wardly 51 July 25, 1972 [54] ELECTROSTATIC PRISM[72] lnventor: George A. Wardly, Yorktown Heights,

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Dec. 23, 1969 21 Appl. No.3 887,603

[52] US. Cl ..250/41.9 ME, 250/49.5 A, 313/63 [51] Int. Cl. ..H01j 39/34[58] Field of Search ..250/4l.9 ME, 49.5 A, 49.5 PE, 250/49.5 C; 3l3/63[56] References Cited UNITED STATES PATENTS 3,309,517 3/1967 Liot........250/4l.9

3,407,323 10/1968 Hand ..250/41.9

3,461,306 8/1969 Stout et al.... .....250/49.5

3,397,311 8/1968 Saari et a1. ..250/4l.9

OTHER PUBLlCATlONS Wardly, Thesis, A Study of Electron BombardmentInduced Conductivity in Thermally Grown Silicone Dioxide, 68

Wardly, Magnetic Contrast Analyzer, 6/69, pg. 298- 299, lBMTDB, Vol. 12,No. 2.

Primary Examiner-Benjamin A. Borchelt Assistant Examiner-N. MoskowitzAttorney-Hanifin and Jancin and John A. Jordan [57] ABSTRACT Anelectrostatic field to manipulate charged particles in a narrow bandfilter prism and energy discrimination prism arrangement is obtained byemploying high transparency opposing screen electrodes. Adjacent eachscreen electrode particle attractor apparatus act to attract allparticles outside the narrow band of particles. In the narrow bandfilter arrangement collector apparatus are provided to collect particleswithin the narrow band while in the discrimination arrangement collectorapparatus are provided to collect both particles having energies abovethe narrow band and particles having energies below the narrow band.

16 Claims, 3 Drawing Figures DETECTOR PATWEBJMS I972 BEST AVAILABLE,COPY 3.679.898

\ SCREEN- DETECTOR 9 31 j r 1 INVENTOR DETECTOR) T +100V GEORGE A.WARDLY I 43 39 l L/i +1oov 37 ELECTROSTATIC PRISM BACKGROUND OF THEINVENTION The present invention relates generally to the manipulation ofcharged particle beams and, more particularly, to electrostatic prismsused for creating an electrostatic field for manipulating and deflectingcharged particles such as electrons and the like.

Electrostatic and magnetic prisms for deflecting electrons and ionizedparticles are known to have wide application as can be seen, forexample, by their use in mass spectrometers, analyzers, electronmicroscopes, discriminators, filters and the like. It appears in mostapplications electrostatic prisms are preferred over magnetic prismsbecause of their good linearity and lack of hysteresis as well as fieldcontainment. One of the main difficulties with known electrostaticprisms, however, lies in the fact that the opposing electrodes used toeffect the field exhibit little or no transparency to the particlesbeing deflected. Accordingly, particles that impinge upon theseelectrodes may either themselves be reflected or alternatively causesecondary or tertiary particles to be generated which particles may inturn impinge upon the electrodes, etc., and finally be sensed as havingan energy quite difierent from what is in fact the case. The effect ofsuch spurious particles may be characterized as noise.

The problem is compounded by the fact that in opposing plate electrodearrangements, a minimum plate width to gap ratio must be maintained toprovide uniform fields between the electrodes. Accordingly, usefulelectrode gaps require the opposing electrode areas to be relativelylarge.

In accordance with the principles of the present invention, the problemof spurious particles in electrostatic prisms, which particles introducea high degree of noise in the output of the device employing the prism,is met by employing high transmission screens as opposing electrodeswith particle attraction apparatus on each side of the screen. Prisms somade may be used, for example, as a narrow band prism or electrostaticprism discriminator.

It is therefore an object of this invention to provide an improvedelectrostatic prism.

It is a further object of this invention to provide an improvedelectrostatic prism employing high transmission screens as opposingelectrodes with particle attraction apparatus adjacent each screen andparticle collection apparatus.

It is yet another object of this invention to provide an improved narrowband electrostatic prism.

It is yet further another object of this invention to provide animproved electrostatic prism discriminator.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

In the drawings:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a preferred embodiment ofthe electrostatic prism in accordance with the present invention in anarrow band prism arrangement.

FIG. 2 shows an example of one form of high transmission screening thatmay be employed in the arrangements of FIGS. 1 and 3.

FIG. 3 shows a preferred embodiment of the electrostatic prism inaccordance with the present invention in a prism discriminatorarrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with theembodiment shown in FIG. 1, an electrostatic prism exhibiting narrowband energy filtering properties with high signal-to-noise ratio isprovided.

As shown in FIG. 1, a high transparency conductive screening is employedas each of the opposing electrodes 1 and 3 to thereby create anelectrostatic field therebetween to effect particle deflection. Thescreen electrodes 1 and 3 are held by any of a variety of supportapparatus, in a planar or plate-type arrangement so that the plane ofthe screen extends from the plane of the paper.

It is to be understood, of course, that other than planar typearrangements are possible.

Where the screen electrodes 1 and 3 are to deflect negatively chargedparticles, as for example electrons, the polarity of the potentialapplied would be as shown i.e., screen electrode 1 positive and screenelectrode 3 negative. It is clear that for deflection of positiveparticles this polarity would be reversed. For the narrow band filterapplication of FIG. 1, a 10 volt magnitude at each electrode wouldprovide an appropriate field for deflecting electrons so that electronsof a selected energy will pass through exit aperture 7 in FIG. 1. Asshown in FIG. 1, the screen electrodes 1 and 3 are arranged to formconcentric arcs of approximately l27 so that the angular path ofparticles from object or source 5 to exit aperture 7 traverses 127 17'.It is clear that the object to image angular distance is chosen tooptimize the focal or discrimination properties of the electrostaticprism. A nominal starting point is to provide 127 17 between object andimage although a non-zero angular aperture and an object lying outsidethe prism can alter the optimum design from the above 127 17.

Any of a variety of finely woven conductive mesh arrangements may beemployed as opposing electrodes in FIGS. 1 and 3 in accordance with thepresent invention. It is clear however, that since the angle ofincidence of the particles is small, the finer the mesh, particularlythe thinner the cross-sectional dimension of the mesh, the higher willbe the transparency.

It is also clear that the greater the spacing between mesh members thehigher will be the transparency. However, it is evident that to achievean effectively uniform field pattern between opposing screens thespacing between individual screen or mesh lines or members in at leastone direction must be small compared to the gap or separation betweenthe screens.

FIG. 2 shows one form of screen exemplary of those that may be employed.It is evident that any ofa variety of conductive materials may be used.For example, gold or a gold plated alloy may be employed to advantagesince it is not susceptible to oxidation and exhibits good electricalproperties. However, it is clear that materials having a lower secondaryemission characteristic than gold may also be employed to advantage.

As shown in FIG. 2, the screen arrangement may be fabricated, forexample, by photoetching a 2-mil thick sheet of phosphor bronze. Supportmembers 21, 23 and 25 may, for example, each be 20 mils. wide and spacedfrom one another by 200 mils. Each of screen lines 27A through 27Q maybe, for example, 2 mils. wide and spaced from one another by 20 mils.After etching, the screen may be plated with gold.

When the screening configuration of FIG. 2 is employed in theelectrostatic prism, in accordance with the present invention, it isarranged so that the particles move in a direction normal to members 21,23 and 25. The wide spacing between these latter members aids inobtaining high transparency while the relatively close spacing betweenlines 27A through 270 insures an effectively uniform field betweenopposing screens. Screening arrangements, akin to that of FIG. 2, havebeen calculated to transmit to percent of a beam of charged particles,such as electrons, approaching the screen at normal incidence, i.e.,orthogonal to the screen surface. At the other extreme, the screen hasbeen calculated to transmit as high as 50 percent of a beam of particlesapproaching the screen within one degree of the tangent to the screen.

In addition to the high transmission screening 1 and 3, narrow bandfilter arrangement of FIG. 1 also employs a pair of particle attractorplates 11 and 13 which may be fabricated from any of a variety ofconductive materials, preferably of low secondary emission yield. Theseplates are each biased with a reasonably large potential so thatparticles, passing through either of the screen electrodes 1 or 3because they are outside the band of energy being selected, areattracted to one or the other of the respective plates 11 or 13 wherethey are suppressed. Likewise, any secondary or tertiary particles whichmay result from collision of the primary particles and the screenelectrodes may also be collected.

As shown in FIG. 1 where the particles being investigated are electrons,biasing each attractor plate 11 and 13 with a +lO-volt potentialprovides an adequate field to attract and hold particles having energiesoutside the selected energy band. It is clear that where positivelycharged particles are being investigated, the polarity of plates 11 and13 would be negative and the magnitude of such polarity would dependupon the particular positively charged particle being considered.

The prism arrangement in accordance with the present invention may behoused in any of a variety of container or chamber apparatus. As shownin FIG. 1, a chamber is employed which acts as a grounded electrostaticand magnetic shield arrangement. Source 5 is a source capable ofproducing a beam of electrons and particle collection detector device 9,which may be any of a variety of conventional electron collectors, suchas a scintillation counter or detector, acts to collect and determineelectrons in the selected energy band. Altematively, a simple electrodearrangement could be employed at 9 to collect and detect particles andthe current produced by the collected particles determined.

It can be seen from FIG. 1 that electrons having an energy greater thanthe selected energy band to be filtered are transmitted through screen 3and attracted to plate 11 where same are suppressed. Likewise, electronshaving an energy less than the selected energy band to be filtered aretransmitted through screen 1 and attracted to plate 13 where same aresuppressed. Accordingly, only electrons having an energy within thenarrow collection band, as defined by the relationship between theparameters of the source 5 and exit aperture 7, are passed to detector 9since the high transmission characteristics of the opposing screenedelectrodes prevents any significant number of electrons outside of theenergy band from creating noise by producing any of reflected, secondaryor tertiary electrons, some of which would be collected by detector 9.

In FIG. 3, there is shown an electrostatic prism discriminator inaccordance with the principles of the present invention. As in the caseof the narrow band filter arrangement in FIG. 1, the wide band energydiscriminator arrangement in FIG. 3 employs high transparency conductivescreening as opposing electrodes 31 and 33 to thereby create anelectrostatic field to effect particle deflection. However, instead ofthe exit aperture 7. as employed in the narrow band filter of FIG. 1,the discriminator of FIG. 3 employs a knife-edge energy leveldiscrimination plate 37 to provide a fine energy interception line fordivision between what may be called the high and low energy particles.Plate 37, which is grounded at one end, may

be fabricated from any of a variety of conductive materials.orthogonally affixed to the end of grounded discrimination plate 37 is asection of high transparency conductive screening 39, akin to screening31 and 33.

The screen electrodes 31 and 33 and screen section 39 may be held by anyof a variety of support apparatus in a planar or plate-type arrangementso that the plane of the screen extends from the plane of the paper. Theangular distance between particle beam source 35 and screen section 39is nominally 127 17'. It should be noted that the function of screensection 39 is to correct for the end effect of screen electrodes 31 and33 whereby the fringe field between these electrodes is not uniform atthe ends thereof. Conductive screen section 39, which is not in physicalcontact with screen electrodes 31 and 33, acts to effect a distributionof the field at the ends of these electrodes so as to provide areasonably uniform field thereat. In addition, screen 39 reduces thepenetration of fields generated by the potentials on screens 41 and 43into the space between screens 31 and 33.

It should be noted that in FIG. 1 a narrow band of particle energies isdetected and the opposing screen electrode arrangement is employed toallow unwanted particles outside this narrow band to be readilytransmitted and suppressed.

However, in FIG. 3, wide bands of particle energies on adjacent sides ofa narrow band are to be detected and the opposing screen electrodearrangement is employed to allow the wanted particles outside thisnarrow band to be transmitted for detection. In either instance, thehigh transmission characteristic of the prism screening reduces theefi'ects of reflected and lower order particles.

Accordingly, in FIG. 3 particle collection detector devices, such as apair of conventional scintillation counters or detectors are employed,as shown at 45 and 47 on each side of the chamber. It is clear, however,that in place of the pair of scintillation counters other forms ofdetectors could be used. For example, a pair of conventional electrodescould be used to measure the current produced by the particles collectedthereat. Surrounding each of counters 45 and 47 are particle attractorscreens 41 and 43, also fabricated from high transmissive screening.Where the discriminator is to be used to discriminate between the highand low energies of negatively charged particles, as for exampleelectrons, establishing a +l00-volt potential, as shown, on each ofattractor screens 41 and 43 provides a sufficient field for attractingthe electrons. Likewise, for electron use, 10 KV scintillation countersmay be used for particle collection and counting and opposing electrodescreens 31 and 33 may be biased to +2.5 volts and 2.5 volts,respectively, as shown.

It is clear that the electrostatic prism discriminator of FIG. 3 may behoused in any of a variety of container or chamber apparatus. As shownin FIG. 3, the grounded chamber 49 is designed so that the top surface51 extends upwardly so as to provide sufficient room for high energyparticles to clear the chamber before being angularly attracted todetector 45.

As can be seen with reference to FIG. 3, particles above a selectedenergy level, are determined by the opposing potential applied to theprism screens 31 and 33, are attracted to scintillation counter 45 whileparticles below the selected energy level are attracted to scintillationcounter 47. A relatively few particles having an energy within thenarrow band of discrimination plate 37 will not be detected.

It is evident that the discrimination arrangement of FIG. 3 may readilybe employed in electron microscopy voltage contrast work. However, otherapplications are likewise readily apparent.

What is claimed is:

1. An electrostatic prism device for creating a field to deflectparticles of a beam of charged particles comprising:

a source for producing a beam of charged particles;

chamber means having an entrance aperture for passing said beam ofcharged particles into said chamber means;

a pair of oppositely poled high transparency screen electrode meansmounted within said chamber means in curved spaced apart relationship tocreate a curved electrostatic field space therebetween extending in anare from said aperture so as to receive therein in said field space saidbeam of charged particles passing through said aperture into saidchamber; and

a pair of particle attractor means each exhibiting a voltage potentialhaving a polarity opposite to that of the polarity of said chargedparticle, one of said pair of particle attrac tor means mounted withinsaid chamber means, exterior to said field space, adjacent one of saidhigh transparency screen electrode means and the other of said pair ofparticle attractor means mounted within said chamber means, exterior tosaid field space, adjacent the other of said high transparency screenelectrode means.

2. The device as set forth in claim 1 wherein said chamber meansincludes exit aperture means at the end of said curved field spaceopposite said entrance aperture so that particles of selected energytraverse an angular path within said curved field space to said exitaperture.

3. The device as set forth in claim 2 wherein particle collection meansact to collect and detect particles of said selected energy exitingthrough said exit aperture means.

4. The device as set forth in claim I further including a pair ofparticle collector means, one of said pair of particle collector meansarranged to collect particles attracted by the said one of said particleattractor means and the other of said particle collector means arrangedto collect particles attracted by the said other of said particleattractor means.

5. The device as set forth in claim 4 further including energy leveldiscrimination plate means positioned at the end of said curved fieldspace opposite to said entrance aperture so as to intercept selectedparticles within a predetermined narrow energy band to thereby dividethe remainder of said particles into high and low energy groups.

6. An electrostatic prism device for creating a field to deflectparticles of a beam of charged particles comprising:

a source for producing a beam of charged particles;

chamber means having an entrance aperture for passing said beam ofcharged particles into said chamber means;

a pair of particle transmissive electrically conductive screening meansmounted within said chamber means in curved spaced apart relationshipextending in an arc from said aperture so as to receive in the spacetherebetween said beam of charged particles passing through saidaperture into said chamber;

means to apply a potential between said screening means to create anelectrostatic field in said curved space therebetween for deflectingsaid beam of charged particles; and

a pair of particle attractor means each biased with a voltage potentialhaving a polarity opposite to that of the polarity of said chargedparticles one of said pair of particle attractor means mounted withinsaid chamber means, exterior to said space, adjacent one of saidscreening means and the other of said pair of particle attractor meansmounted within said chamber means, exterior to said space, adjacent theother of said screening means.

7. The device asset forth in claim 6 wherein each of said pair ofparticle attractor means is in the form of a section of particletransmissive electrically conductive screening means and wherein saiddevice further includes a pair of particle collection means with one ofsaid pair of particle collection means mounted to collect particlespassing through one of said pair of screening means and with the otherof said pair of particle collection means mounted to collect particlespassing through the other of said pair of screening means.

8. The device as set forth in claim 6 further including a groundedenergy level discrimination plate positioned within said chamber meansat the end of said space opposite to said entrance aperture to interceptselected ones of said particles having a predetermined narrow energyband to thereby divide the remainder of said particles into high and lowenergy groups.

9. The device as set forth in claim 6 wherein said particles areelectrons.

[0. The device as set forth in claim 6 wherein said screening means areparticle transmissive up to 50% for particles approaching said screenwithin one degree of the tangent to the screen.

11. The device as set forth in claim 6 wherein said pair of particleattractor means each comprise a conductive plate of low secondaryemissive material which acts to attract and hold particles in accordancewith the energy of the said particles.

12. The device as set forth in claim 11 wherein said chamber meansincludes an exit aperture in said chamber at the end of said spaceopposite to said entrance aperture and wherein collector means areprovided to collect particles having energies falling within the narrowband of energies required for particles to pass through said exitaperture.

13. The device as set forth in claim 12 wherein said particles areelectrons.

14. A narrow band electrostatic prism device for filtering particles ofa selected narrow band of energy from a beam of charged particlescomprising:

source means for producing a beam of charged articles;

chamber means having an entrance aperture or receiving said beam ofcharged particles into said chamber and an exit aperture angularlydisplaced from said entrance aperture for exiting selected ones of saidparticles entering said chamber;

electrostatic field forming means mounted within said chamber means forforming an electrostatic field extending in an angular path between saidentrance aperture and said exit aperture, said field forming meansincluding a pair of oppositely poled high transmission screeningelectrode means mounted in spaced apart relationship to form an angularfield space extending from said entrance aperture to said exit aperture;

a pair of particle attractor means each exhibiting a voltage potentialhaving a polarity opposite to that of the charge of the particles ofsaid beam of charged particles, one of said pair of particle attractormeans mounted within said chamber means, exterior to said field space,adjacent one of said high transmission screening electrode means forattracting high energy particles and the other of said pair of particleattractor means mounted within said chamber means, exterior to saidfield space, adjacent the other of said high transmission screeningelectrode means for attracting low energy particles; and

particle collection means for collecting and detecting particles havingenergy within the narrow band of energy required to effect traversal ofa trajectory within said angular field space to said exit aperture.

15. An electrostatic prism energy level discriminator for discriminating between the high energy and low energy particles of a beamof charged particles comprising:

source means for producing a beam of charged particles;

chamber means having an entrance aperture for receiving said beam ofcharged particles into said chamber;

electrostatic field forming means mounted within said chamber means forforming an electrostatic field extend ing in an angular path from saidentrance aperture, said field forming means including a pair ofoppositely poled high transmission screening electrode means mounted inspaced apart relationship to form an angular field space extending fromsaid entrance aperture;

particle interception plate means mounted at the end of said angularpath opposite to said entrance aperture so as to intercept particleswithin a selected narrow energy band;

a pair of particle attractor means each exhibiting a voltage potentialhaving a polarity opposite to that of the charge of the particles ofsaid beam of charged particles, one of said pair of particle attractormeans mounted within said chamber means, exterior to said field space,adjacent one of said high transmission screening electrode means forattracting high energy particles and the other of said pair of particleattractor means mounted within said chamber means, exterior to saidfield space, adjacent the other of said high transmission screeningelectrode means for attracting low energy particles; and

a pair of particle collection means for collecting and detecting saidparticles, one of said pair of particle collection means arranged tocollect and detect high energy particles attracted by the said one ofsaid pair of particle attractor means and the other of said particlecollection means arranged to collect and detect low energy particlesattracted by the said other of said particle attractor means.

16. The discriminator as set forth in claim 15 wherein said pair ofparticle attractor means each comprise high transmission screening meansrespectively positioned between its corresponding particle collectormeans and said electrostatic field forming means.

1. An electrostatic prism device for creating a field to deflectparticles of a beam of charged particles comprising: a source forproducing a beam of charged particles; chamber means having an entranceaperture for passing said beam of charged particles into said chambermeans; a pair of oppositely poled high transparency screen electrodemeans mounted within said chamber means in curved spaced apartrelationship to create a curved electrostatic field space therebetweenextending in an arc from said aperture so as to receive therein in saidfield space said beam of charged particles passing through said apertureinto said chamber; and a pair of particle attractor means eachexhibiting a voltage potential having a polarity opposite to that of thepolarity of said charged particle, one of said pair of particleattractor means mounted within said chamber means, exterior to saidfield space, adjacent one of said high transparency screen electrodemeans and the other of said pair of particle attractor means mountedwithin said chamber means, exterior to said field space, adjacent theother of said high transparency screen electrode means.
 2. The device asset forth in claim 1 wherein said chamber means includes exit aperturemeans at the end of said curved field space opposite said entranceaperture so that particles of selected energy traverse an angular pathwithin said curved field space to said exit aperture.
 3. The device asset forth in claim 2 wherein particle collection means act to collectand detect particles of said selected energy exiting through said exitaperture means.
 4. The device as set forth in claim 1 further includinga pair of particle collector means, one of said pair of particlecollector means arranged to collect particles attracted by the said oneof said particle attractor means and the other of said particlecollector means arranged to collect particles attracted by the saidother of said particle attractor means.
 5. The device as set forth inclaim 4 further including energy level discrimination plate meanspositioned at the end of said curved field space opposite to saidentrance aperture so as to intercept selected particles within apredetermined narrow energy band to thereby divide the remainder of saidparticles into high and low energy groups.
 6. An electrostatic prismdevice for creating a field to deflect particles of a beam of chargedparticles comprising: a source for producing a beam of chargedparticles; chamber means having an entrance aperture for passing saidbeam of charged particles into said chamber means; a pair of particletransmissive electrically conductive screening means mounted within saidchamber means in curved spaced apart relationship extending in an arcfrom said aperture so as to receive in the space therebetween said beamof charged particles passing through said aperture into said chamber;means to apply a potential between said screening means to create anelectrostatic field in said curved space therebetween for deflectingsaid beam of charged particles; and a pair of particle attractor meanseach biased with a voltage potential having a polarity opposite to thatof the polarity of said charged particles one of said pair of particleattractor means mounted within said chamber means, exterior to saidspace, adjacent one of said screening means and the other of said pairof particle attraCtor means mounted within said chamber means, exteriorto said space, adjacent the other of said screening means.
 7. The deviceasset forth in claim 6 wherein each of said pair of particle attractormeans is in the form of a section of particle transmissive electricallyconductive screening means and wherein said device further includes apair of particle collection means with one of said pair of particlecollection means mounted to collect particles passing through one ofsaid pair of screening means and with the other of said pair of particlecollection means mounted to collect particles passing through the otherof said pair of screening means.
 8. The device as set forth in claim 6further including a grounded energy level discrimination platepositioned within said chamber means at the end of said space oppositeto said entrance aperture to intercept selected ones of said particleshaving a predetermined narrow energy band to thereby divide theremainder of said particles into high and low energy groups.
 9. Thedevice as set forth in claim 6 wherein said particles are electrons. 10.The device as set forth in claim 6 wherein said screening means areparticle transmissive up to 50% for particles approaching said screenwithin one degree of the tangent to the screen.
 11. The device as setforth in claim 6 wherein said pair of particle attractor means eachcomprise a conductive plate of low secondary emissive material whichacts to attract and hold particles in accordance with the energy of thesaid particles.
 12. The device as set forth in claim 11 wherein saidchamber means includes an exit aperture in said chamber at the end ofsaid space opposite to said entrance aperture and wherein collectormeans are provided to collect particles having energies falling withinthe narrow band of energies required for particles to pass through saidexit aperture.
 13. The device as set forth in claim 12 wherein saidparticles are electrons.
 14. A narrow band electrostatic prism devicefor filtering particles of a selected narrow band of energy from a beamof charged particles comprising: source means for producing a beam ofcharged particles; chamber means having an entrance aperture forreceiving said beam of charged particles into said chamber and an exitaperture angularly displaced from said entrance aperture for exitingselected ones of said particles entering said chamber; electrostaticfield forming means mounted within said chamber means for forming anelectrostatic field extending in an angular path between said entranceaperture and said exit aperture, said field forming means including apair of oppositely poled high transmission screening electrode meansmounted in spaced apart relationship to form an angular field spaceextending from said entrance aperture to said exit aperture; a pair ofparticle attractor means each exhibiting a voltage potential having apolarity opposite to that of the charge of the particles of said beam ofcharged particles, one of said pair of particle attractor means mountedwithin said chamber means, exterior to said field space, adjacent one ofsaid high transmission screening electrode means for attracting highenergy particles and the other of said pair of particle attractor meansmounted within said chamber means, exterior to said field space,adjacent the other of said high transmission screening electrode meansfor attracting low energy particles; and particle collection means forcollecting and detecting particles having energy within the narrow bandof energy required to effect traversal of a trajectory within saidangular field space to said exit aperture.
 15. An electrostatic prismenergy level discriminator for discriminating between the high energyand low energy particles of a beam of charged particles comprising:source means for producing a beam of charged particles; chamber meanshaving an entrance aperture for receiving said beam of charged particlEsinto said chamber; electrostatic field forming means mounted within saidchamber means for forming an electrostatic field extending in an angularpath from said entrance aperture, said field forming means including apair of oppositely poled high transmission screening electrode meansmounted in spaced apart relationship to form an angular field spaceextending from said entrance aperture; particle interception plate meansmounted at the end of said angular path opposite to said entranceaperture so as to intercept particles within a selected narrow energyband; a pair of particle attractor means each exhibiting a voltagepotential having a polarity opposite to that of the charge of theparticles of said beam of charged particles, one of said pair ofparticle attractor means mounted within said chamber means, exterior tosaid field space, adjacent one of said high transmission screeningelectrode means for attracting high energy particles and the other ofsaid pair of particle attractor means mounted within said chamber means,exterior to said field space, adjacent the other of said hightransmission screening electrode means for attracting low energyparticles; and a pair of particle collection means for collecting anddetecting said particles, one of said pair of particle collection meansarranged to collect and detect high energy particles attracted by thesaid one of said pair of particle attractor means and the other of saidparticle collection means arranged to collect and detect low energyparticles attracted by the said other of said particle attractor means.16. The discriminator as set forth in claim 15 wherein said pair ofparticle attractor means each comprise high transmission screening meansrespectively positioned between its corresponding particle collectormeans and said electrostatic field forming means.